Linear prediction coding (LPC) is a digital signal processing technique which makes possible the transmission of digitized voice signals at comparatively low bit rates. Conventional digitized voice (PCM, delta modulation) requires transmission bit rates of 20 kbps and above for good quality. LPC techniques permit good quality performance with transmission bit rates in the range of 1400 to 8000 bits per second. Bandwidth reductions such as this have been accomplished previously by other types of vocoders. Previous vocoders, however, tend to not produce natural sounding speech, and tend to be large and expensive. LPC has been demonstrated to produce good sounding speech. With the present state of LSI technology and LPC vocoders, LPC implementation requires 20 to 50 LSI chips, which is a significant reduction in size, weight, and cost in quantity, but is still a large amount of circuitry. The attractiveness of LPC, compared with other vocoder approaches, is that it is digital in nature, i.e., it is performed by a digital computer of some sort, and the cost of computation has fallen as LSI technology has advanced. However, LPC vocoders, prior to the present invention, are still relatively expensive.
Vocoders are based on the fact that the information content of speech has a bandwidth much less than the bandwidth of the speech signal itself. Vocoders, and LPC in particular, measure certain parameters of the speech signal, and transmit these parameters over a communication channel (at a bandwidth less than that required for the speech signal itself). At the receiver, these parameters are used to reconstruct, or synthesize, a signal which (ideally) sounds like the original speech.
The LPC analyzer (coder) takes the form of a transversal filter with complex weights. The speech signal is passed through a tapped delay line. The output of each tap is weighted and summed together with all of the other weighted tap outputs in a final summer. The weight values are adjusted so as to minimize the error signal at the output of the final summer. The method by which the weight values are determined defines the LPC algorithm. To date, these algorithms have all used digital filtering techniques. Because each tap of the delay line requires a weighter and circuitry for determining the weight values, a great amount of circuitry is required for the analyzer, which greatly increases the expense and complexity. Further, the synthesizer, or decoder, at the other end of the communication network must essentially reverse the procedure and, therefore, requires approximately the same amount of components.